1. Field of Invention
This invention pertains generally to etching and, more particularly, to a very fast etching system and process for use in the manufacture and backside etching of silicon wafers, integrated circuit packaging, and the manufacture of circuit boards.
2. Related Art
Historically, reactive ion etching, the prevalent method of plasma-based etching processes for integrated circuit (IC) manufacture, has used radio frequency electrical discharges between substantially parallel electrodes. The discharge produces ions and neutral reactive atoms and molecules that are responsible for the etching action. These etching processes were typically used in IC fabrication for silicon, silicon dioxide, silicon nitride or aluminum removal and used reactant gases containing fluorine or chlorine. Such processes have usually been anisotropic etching processes in which the material to be patterned was removed with the boundary being a plane substantially perpendicular to the wafer surface defined by a photolithographic mask. The typical removal rate of material for these processes was several thousand Angstroms per minute, adequate for the purposes of integrated circuit manufacture. The mask is made of photosensitive material—which is an organic polymer called photoresist. This etching process was called reactive ion etching (RIE) because it was the ions which provided the activation energy for the etching reactions, and the ions usually contained halogen atoms which formed volatile species upon reaction with the exposed material on the wafer. Under the conditions of the process, the ions from the plasma impact the wafer nearly perpendicular to the wafer surface activating reactions mostly on surfaces which are substantially parallel to the wafer surface and avoiding etching on surfaces which are perpendicular to the wafer surface.
The etching rate for these processes is adequate for the thin films used in IC fabrication which are usually no more than a few microns thick. (It is important that these etching processes not require too much time for completion in order that the cost of the processing not be excessive.) Typical system costs for such etching tools is about two to four million dollars. One reason for the high cost and low speed of such IC etching is that the structures to be etched on wafers are typically somewhat less than one micron in critical lateral dimension and therefore the process must be very clean. Other reasons for the high cost are that such processes must carefully control the angle of the sidewall produced by the etching and must strictly control the contaminant materials which are transferred to the surface during the etching process since very minute amounts will destroy the semiconductor's function.
The method of formation of a plasma for RIE is that radio frequency power is provided to the pedestal upon which the wafer is placed or the showerhead/upper electrode above the wafer. In addition to causing electrical breakdown and ionization of the gas it causes there to be a dc potential difference between the pedestal and the plasma which accelerates the positive ions to the wafer surface. The ions are thus given sufficient energy to promote the etching processes that is desired. The ions do this by activating the etching reactions of the halogen species with the exposed material on the wafer surface and also causing the reaction products to come off the surface either by sputtering or desorption. (J. W. Coburn and H. F. Winters, Plasma Etching—A discussion of mechanisms; Journal of Vacuum Science and Technology, Vol 16, page 391-403, 1979) One of the other reasons for the effectiveness of this technique was that if impurities which do not form volatile products built up on the surface, the ions from the plasma would sputter them off and thereby allow the etching to continue. Typical ion energies in this type of discharge are of order 100 eV to several hundred eV.
The typical gases tried for such etching processes of silicon or silicon-based insulators or alloys have included halogenated gases of many types. The typical current processes for etching silicon itself in IC fabrication have mainly included chlorine and bromine based gases. The reason is that the etching needs to be anisotropic and therefore there is a need for halogens which do not etch silicon unless ion impact promotes the necessary reactions. Sometimes, fluorocarbon gases may be used but they have a strong tendency to etch the silicon isotropically and undercut the patterning mask. On the other hand, isotropic etching processes for silicon, based on plasma sources, are usually done using fluorocarbons or nitrogen trifluoride as reactant gases. One reason for this is sulfur hexafluoride is generally considered less efficient than nitrogen trifluoride for producing such isotropic silicon etching and may leave sulfur containing residues. Etching of silicon with sulfur hexafluoride was tried in the early days of plasma etching by various researchers and found to be unexceptional in its properties, and was not adopted as principal halogenated gas for etching processes. (K. M. Eisele, Journal of the Electrochemical Society, Vol. 128, page 123-126, 1981; also W. Beinvogl, H. R. Deppe, R. Stokan and B. Hasler, I.E.E.E. Transactions on Electron Devices, Vol. ED-28, page 1332-1337, 1981; and Anisotropic etching of polysilicon using SF6 and CFCl3, M. Mieth and A. Barker, J. Vac. Sci. Technol. A1, 629-635, 1983) These researchers disclosed etching processes with rates well below one micron per minute typically with gas pressures of several hundred millitorr and power levels up to several watts per square centimeter.
The RIE process, however, has not been found suitable for some etching steps in semiconductor fabrication because of the energy of the ions which impact the wafer and the active electrical charging it causes on the surface of the wafer. Included among these are processes where sensitive areas of the mono-crystalline silicon are exposed and therefore there could be dislocations (resulting in impaired function) caused by the ion impact. Among such processes are those for etching organic contaminants left on the wafer after other patterning processes and in which no directionality of the etch is desired. Such processes are called isotropic. Mattson and Martin (U.S. Pat. No. 5,198,634) demonstrated the usefulness of a new regime for isotropic etching of organic polymers using the parallel plate discharge in which the gas pressure in the process chamber was much higher than in previous work. In their process the radio frequency power input was also limited in proportion to the pressure and the volume of the plasma (less than 0.15 Watts/Cm with the intent of reducing electrical and ion impact damage to the semiconductor devices located on the wafer surface. Their etching technique permitted a relatively high rate of removal of organic residues or other undesired material from the surface of a semiconductor wafer while not causing electrostatic charge-based damage to the sensitive transistors being fabricated on the wafer surface. The pressure for their process was so high (typically 20 Torr to 30 Torr) that the ions from the plasma make many collisions in moving to the wafer and only have a few eV of energy remaining when they impact the wafer. However, the Mattson processing system relies for its efficiency of etching (typically of organic material) on the use of higher wafer temperature (>150 C and usually about 250 C) to achieve their rate of etching. Their invention successfully achieves the goals of avoiding charging and ion damage while getting high removal rates for the organic photoresist—usually about two microns per minute. It also avoids, as required, sputtering the exposed materials of the semiconductor devices or interconnects on the wafer surface. This apparatus, however, is not capable of removing organic materials at high rates when the wafer temperature is low (<100° Celsius) nor can it achieve rapid etching of silicon or silicon-based materials. At temperatures below 100° Celsius the etching rate drops by at least an order of magnitude to about a few thousand Angstroms per minute. High rate etching of materials at low temperature requires a larger supply of chemically reactive species, such as oxygen or fluorine atoms than the Mattson device can supply, and an alternative source of activation energy to be provided for the desired etching reactions at the surface of the substrate. In RIE that energy is substantial and causes sputtering of materials and even crystalline damage to the silicon of the wafer. In other applications such ion bombardment would cause sputtering of some of the materials exposed on the workpiece which would cause problems for the finished product.
In applications where crystalline silicon is to be etched isotropically at high rates there have been reports of such processes succeeding without benefit of ion bombardment but these have used very high density plasma torches in which the gaseous species—particularly fluorine atoms—are at high temperatures. In such plasmas the heat transfer to the wafer by the gas is very considerable. In this case there is a requirement for very high speed scanning of the plasma across the substrate to avoid heating damage. (S. Savastiouk, O. Siniaguine and M. Hammond, Atmospheric Pressure Downstream Plasma—a new tool for semiconductor processing, Solid State Technology, June, 1998.) If such etching is to be done at modest temperatures as required for some applications the heating associated with this high pressure torch method needs to be avoided. Further, the high speed scanning and intense heat removal make reliability of such a processing system hard to achieve.
U.S. Pat. No. 5,198,634 describes a plasma reactor for contamination removal using a much lower ratio of power density to gas pressure. That system is not capable of the high rate etching needed under the temperature conditions which are required.